When a band-shaped blade is used to punch a sheet to a desired shape, it is mounted on a base, and fitted in a groove formed therein to the desired shape. In order to enable the band-shaped blade to fit in the groove, the blade should be previously bent at an obtuse angle, an acute angle or right angle depending upon the shapes of the groove and the radius of curvatures as shown in FIG. 8 where the full line indicates the contour to be punched along and the dotted lines indicate the foldable lines.
There are at least four known methods of bending a bandwork; first, by hand with a special tool; second, by means of three rollers (FIG. 9A); third, by means of a pair of molds each having a required radius of curvature where the work held between the molds is punched under a single blow (FIG. 9B), and fourth, by gradually bending the bandwork while it is fed through a pair of chips inch by inch (FIG. 9C).
The last-mentioned method encounters two difficulties; one is that when a bandwork is bent at one spot at an obtuse angle, it often happens that the subsequent bending is difficult by bringing the already bent portion of the bandwork with the apparatus, and the other is that accumulated dimensional errors eventually fails to achieve the intended accuracy.